What is the name of this 80's movie?

There is a movie about a guy who meets a blonde girl and likes her but he turns into a monster at night. There were people getting murdered and he didn't know if it was him doing the killings. There turned out to be this real dorky character in the movie who also was a monster who turned out to be the one doing the killing. Theres a part where the main guys head explodes and all this confetti comes out and it freaks out the blond. Her best friend is a latina chick who happened to get attacked also. And there is a psycic that has a crystal ball in his brief case who gives the main guy info.